1. Field of the Invention
The present invention relates to an image transmitting device for use in an electrophotographic copying machine, image reading machine, or the like and, more particularly, it relates to a control system for controlling the reciprocation of a scanning arrangement, wherein the scanning arrangement scans an original document in a programmed manner so as to sequentially transmit images of the original document in slits onto a photoreceptor surface.
2. Description of the Prior Art
Generally, an electrophotographic copying machine or an image reading machine has a platen or a support for holding an original document, and a scanning arrangement for scanning across the original document in slits. The scanning arrangement includes a source of light producing light beams in a form of a ribbon that impinges on the original document and an image transmitting device for transmitting the image, i.e., reflected light beams from the original document to the photoreceptor surface. The image transmitting device is defined by a pair of mirror devices, or a movable lens device (such as disclosed in U.S. Pat. No. 3,584,950 to Gundlach), and a driving device for driving such a transmitting device. When the transmitting device is defined by two mirror devices, the driving device moves the mirror devices in predetermined different speeds. For example, the two mirror devices move at different speeds having a ratio of 2 to 1, thereby effecting a complete scan over of the original document and, thereafter, returning the scanning arrangement back to its initial position.
According to the prior art, the scanning arrangement is coupled to a main motor through a suitable clutch device which may take one of three different positions: a neutral position; a forward transmitting position; and a reverse transmitting position. When the clutch device is in the neutral position, no driving force is transmitted to the scanning arrangement.
When the clutch device is in the forward transmitting position, the scanning arrangement is moved at the constant speed for effecting a scanning operation. More specifically, when the clutch device is shifted from the neutral position to the forward transmitting position, the scanning arrangement starts to move from its initial or home position, and its speed is accelerate until it reaches a predetermined constant speed. The acceleration of the scanning arrangement is carried out in a predetermined distance, hereinafter referred to as a pre-running distance. During the acceleration, the speed of the scanning arrangement is not constant, thus it is not suitable to carry out the image transmission during this period. When the scanning arrangement moves past the pre-running distance, it is moved at a constant speed for carrying out the image transmission from the original document to the receiving surface.
When the scanning arrangement completes one scan, the clutch device is shifted to the reverse transmitting position, thereby returning the scanning arrangement back to its initial position as quickly as possible ready for the next scanning operation.
In recent years many approaches have been made to improve the copying machine, such as to speed up the copying operation, to make the copy as precise as the original, and to minimize the copying machine. To accomplish such approaches, it is required to present an improved scanning arrangement that can scan fast with a high stability and a high preciseness, and that returns back to the initial position with less deviation. Furthermore, it is required to make the pre-running distance as short as possible.
To speed up the copying operation, the returning speed is made much faster than the scanning speed. But, when this is done, it is necessary to give a braking effect in a highly controlled manner, or otherwise, the scanning arrangement stops at a position highly deviated from its home position. If the scanning arrangement fails to return precisely to its initial position, it is necessary to provide means for setting the scanning arrangement to the home position, or to provide a wide home position so that the scanning arrangement has a pre-running distance sufficiently long to accelerate the scanning arrangement to the required speed even when the scanning arrangement is stopped at the position much deviated towards the scanning area. The employment of the setting means or the long prerunning distance results in increase of manufacturing parts or the size of the copying machine. Furthermore, it fails to speed up the copying operation.
To overcome the above disadvantages, a various improved systems have been proposed, such as employing a servo-control system having a pulse motor or a D.C. motor, and such improvements are disclosed in, for example: U.S. Pat. No. 4,332,461 to Cail et al.; U.S. Pat. No. 4,287,461 to Promis et al.; and U.S. Pat. No. 4,371,254 to Beery.
In the proposed systems, however, a memory means is provided which stores information of proper speed and position of the scanning arrangement throughout the operation, and a servo-control that controls the actual speed and position to be the same as those stored in the memory means. The system according to Beery has a control means that controls only the return movement of the scanning arrangement, wherein the speed and position are controlled by the use of grating.
The prior art systems have disadvantages such that a memory means having a large capacity must be used, and that the structure is complicated.
Another improved system is proposed and it is disclosed in, e.g., U.S. Pat. No. 4,280,763 to Arai et al. wherein a train of pulses are generated at a pitch corresponding to the speed of the scanning arrangement, and such a train of pulses are used not only for detecting the speed of the scanning arrangement but also for detecting the position of the scanning arrangement by the use of a suitable counting means. According to one embodiment of Arai, a disk having slits formed therearound is provided for intermittently cutting a light beam, thereby producing a train of pulses. According to another embodiment of Arai, a disk having small magnets mounted therearound is provided of producing a similar train of pulses. In either one of the embodiments, it is preferable to make the frequency of the pulses as high as possible so as to improve the detecting accuracy. To this end, one may take a method either or both of:
(1) narrowing the pitch of the slits or magnets; and/or
(2) providing means for detecting both the leading edge and trailing edge of each pulse.
According to the method (1), the detecting accuracy cannot be improved more than a certain degree, because of mechanical restriction. Also, according to the method (2), the detecting accuracy can be improved only when the duty ratio of the pulses is made exactly 50%, and this requirement is very difficult to meet from the mechanical and electrical point of view.